It is known that when a combustion mode of an engine is switched, a composition of an exhaust gas changes. Patent Document 1, for example, describes that an exhaust gas in a spark ignition (SI) combustion mode contains a large amount of unsaturated hydrocarbon such as olefin but in a homogeneous charge compression ignition (HCCI) combustion mode, the amount of saturated hydrocarbon (pentanes) and CO contained in the exhaust gas increases.
Patent Document 1 describes that in a case where a Pt/Rh mass ratio of a catalyst closely coupled to an exhaust manifold of an engine is increased to 4 to 10 times as high as that of a catalyst under the floor, even when the combustion mode is switched to a homogeneous charge compression ignition combustion mode, saturated hydrocarbon and CO can be efficiently purified. This close-coupled catalyst includes catalyst layers of an upper layer and a lower layer formed on a cell wall of a honeycomb substrate, the upper layer contains a Pt catalyst and a Rh catalyst, and the lower layer contains a Pt catalyst and a Pd catalyst. The Pt catalyst of the upper layer is a Pt-loaded La alumina composite oxide, and the Rh catalyst is a combination of a Rh-loaded CeZrNd composite oxide and a Rh-loaded LaZr/La alumina composite oxide. The Pt catalyst of the lower layer is a Pt-loaded La alumina composite oxide, and the Pd catalyst is a combination of a Pd-loaded CeZrNd composite oxide and a Pd-loaded La alumina composite oxide.